Idiopathic or primary diabetes mellitus is a chronic disorder of carbohydrate, fat, and protein metabolism characterized in its fully expressed form by an absolute or relative insulin deficiency, fasting hyperglycemia, glycosuria, and a striking tendency toward development of atherosclerosis, microangiopathy, nephropathy, and neuropathy. Underutilization of glucose is characteristic of all diabetic patients, but only some have a clearly defined severe insulin deficiency resulting from a loss of beta cells. The large remainder of diabetic patients suffers from some impairment of insulin secretory response associated with a marked resistance to insulin in the peripheral tissues.
The phrase “idiopathic diabetes mellitus” embraces a heterogeneous group of disorders having in common the above-described characteristics. At least two major as well as several less common variants of the disease have been identified. One major variant, insulin-dependent diabetes mellitus (IDDM) (Type I), accounts for about 10% of diabetics. A second major variant,non-insulin-dependent diabetes mellitus (NIDDM) (Type II) represents the remaining 90% of all diabetic patients. Absent regular insulin replacement therapy using exogenously produced insulin and/or careful monitoring of the diet of diabetic patients, such patients experience a wide range of debilitating symptoms, some of which can progress into coma and ultimately death.
In mammals, the pancreas is the primary organ responsible for the maintenance of normoglycemia. Generally, the mature mammalian pancreas develops from 2 pancreatic buds (or anlagen) called the dorsal pancreas and the ventral pancreas. These anlagen will fuse during development to form the pancreas although the dorsal anlage arises first and generates most of the pancreas. The ventral anlage arises beside the bile duct and forms part of the head and uncinate process of the pancreas.
The mature pancreas has both exocrine (digestion) and endocrine (hormonal) functions. The exocrine function includes secreting enzymes to aid in digestion. The pancreatic hormonal function includes secreting at least insulin and glucagon, two hormones which together help regulate blood glucose levels. Within the endocrine pancreas, it is the beta cells, which are organized into areas called islets of Langerhans, that create and excrete insulin. Glucagon is secreted by alpha cells within the islets of Langerhans.
Many attempts have been made to replace pancreatic mass and/or function in diabetic recipients through surgical methods. For example, transplantation of digested and isolated islets of Langerhans derived from human cadaveric pancreas to immunosuppressed diabetic humans is an established but experimental means to treat diabetes mellitus. Given existing technology, a major limitation of this technique is the insufficient supply of human pancreatic tissue available for transplantation. In addition, islet tissue may be lost or degraded during the digestion and isolation procedures as well as after transplantation as only a fraction of the transplanted islets engraft in the host. Mass increase of the beta cells within the islets may also be sub-optimal as such transplants exhibit limited potential for expansion of the beta cell mass. Moreover, the immunosuppression and related issues involved in any cadeveric transplantation can be quite significant.
Isotransplantation of whole immature, fetal rat pancreas to rat subcapsular kidney space, anterior eye chamber, testis, subcutaneous pocket, third ventricle, and cheek pocket, are also known in the art. Such work also includes the effect of insulin treatments on the growth and differentiation of the transplanted tissue.
Transplantation of collagenase digested and isolated mature rat and hamster islets into hamster skin folds and striated muscle tissue wherein the isolated islets revascularize is also known. At least two immunosuppressive regimens, cyclosporineA (CsA) and 15-deoxyspergualin (DOS), are known to be useful for such xenografts. Similarly, it is known to inject dispersed developing rat pancreas (minced, disassociated and collagenase digested) into the peritoneal cavity or subcapsular site of the kidneys of alloxan diabetic rats, to reverse, at least temporarily, their diabetes.
Isolated islet clusters of fetal porcine pancreas have also been used as a xenogeneic transplant for human diabetic patients. Thus, it is known to isolate pancreatic islets from pig fetuses and inject the islets into a vein of a human diabetic patient. A notable problem with such procedures is the necessity for maintenance immunosuppression with cyclosporine, prednisolone and azathioprine. An additional problem is that such grafts do not help the recipient attain control for levels of circulating glucose.
The shortcomings of the prior methods are significant. Dispersed or cluster cell methods require large amounts of donor tissue and usually multiple donors per recipient. Strong immune suppression is also needed to help avoid acute rejection of the transplanted cells. Moreover, given the dispersed form of cellular and cluster transplants, the actual implantation may be difficult to control and no single, developing chimeric organ is ever created. Among other differences, prior transplantations of whole adult or immature pancreatic tissue have utilized tissue substantially more developed than that of the present invention and have not focused on the peritoneum as the transplant site. As a result, the prior techniques exhibit increased risk of hyperacute and/or acute vascular rejections as well as the potential for unwanted and unnecessary development of exocrine pancreatic function. Only the present invention provides for the development of a newly vascularized, chimeric pancreatic organ with endocrine, but not exocrine, function while substantially avoiding or reducing at least hyperacute and acute-vascular transplant rejection.
The chimeric pancreas of the present invention substantially increases or is at least capable of increasing in functional mass within the host to establish at least near normal levels of glycemia within the host. This is reflected by at least: 1) the absence of insulin in immature pancreatic tissue transplants at the time of implantation and the presence of such secreted insulin in developing islets two weeks after transplantation; and 2) the approximately 30 day delay between transplantation of immature pancreatic tissue in streptozotocin diabetic rats and the resulting euglycemia of the host as the tissue develops and matures.
Accordingly, and in light of the failure of any of the prior methods to lead to an effective treatment for diabetes mellitus in humans, there is a need to develop novel methods, tissues, and compositions for transplanting immature pancreatic tissue into mammalian recipients to increase the pancreatic mass of the recipient.